FIBROUS CONNECTIVE TISSUE. 71 



precipitate, in the excess of the reagent, for example, which 

 may even prevent its appearance altogether if the conditions 

 are not favorable. 



Pure gelatin gives the biuret or Piotrowski's reaction 

 strongly, the shade of color being a violet-purple. Millon's 

 and the xanthoproteic reactions are obtained with great 

 distinctness, the former being especially strong, although in 

 both cases the reaction is by no means as marked as that 

 obtained with corresponding amounts of albumin or globulin. 

 The reactions are altogether too pronounced to be credited to 

 traces of contained albumin, as has frequently been done in 

 the past. Far more plausible is it to assume that the radical 

 or radicals (perhaps an oxybenzene group) to which these 

 reactions are due, and which are so abundant in the proteid 

 molecule, are contained only in small proportion in the gelatin 

 molecule. 



With Adamkiewicz's reaction, and also with Liebermann's 

 reaction, pure gelatin yields negative results. 



With acetic acid and potassium ferrocyanide a slight pre- 

 cipitation is obtained, if the ferrocyanide solution is added 

 carefully. A very slight excess of the latter redissolves the 

 precipitate. 



With picric acid, trichloracetic acid, tannic acid, chloro- 

 platinic acid, phosphomolybdic acid and phosphotungstic acid, 

 pure gelatin is more or less completely precipitated. With 

 picric acid the precipitate first formed redissolves readily, 

 becoming permanent on addition of more of the reagent. It 

 is somewhat soluble in excess of the precipitant. With 

 trichloracetic acid the precipitation is far from complete. 



The following reagents do not precipitate pure gelatin: 

 plumbic acetate, basic lead acetate, cupric sulphate, ferric 

 chloride, silver nitrate, and mercuric chloride. 



Mercuric chloride, however, causes a precipitate if hydro- 

 chloric acid is added. As solutions of mercuric chloride some- 

 times contain a little free acid, a turbidity may be obtained 

 with the mercuric chloride alone. The precipitate formed on 

 adding acid is very soluble in excess of acid, especially on 



